1. Field of the Invention
The present invention relates to a semiconductor device for use as an amplifier, a regulator, or the like that requires a phase compensation capacitor to be connected to an output terminal arranged thereof.
2. Description of the Prior Art
Among conventional semiconductor devices requiring a phase compensation capacitor to be connected to an output terminal thereof, a power IC (Integrated Circuit) is taken as an example and described hereinafter. FIGS. 3 and 4 are circuit diagrams showing the outline of a conventional power IC. A power IC 10xe2x80x3 as shown in FIG. 3 is a semiconductor device that sends out signals output from a driver circuit 11 to a load Z1 via an output terminal T1. The driver circuit 11 comprises, for example, multiple amplifying stages (not illustrated).
At the same time, a phase compensation capacitor C1 is connected to the output terminal T1 externally in parallel with the load Z1. This phase compensation capacitor C1 is connected in order to suppress abnormal oscillations occurring in the driver circuit 11 and serves to form a pole at a lower frequency where a phase lag in output signals occurs so as to maintain a loop gain below 1 at a zero (phase=0) which is found in an input-output phase characteristic. This arrangement eventually serves to compensate the phase by decreasing the gain at a higher frequency side.
In the conventional cases, electrolytic capacitors or tantalum capacitors having a capacitance of approximately 10 xcexcF are often used as a phase compensation capacitor. However, since these electrolytic capacitors have a polarity, an accidental connection to an opposite pole when mounting the capacitor induces problems that cause the capacitor to emit smoke or break down. Recently, because of these drawbacks, nonpolarized capacitors, particularly ceramic capacitors, have been used preferably as a phase compensation capacitor instead.
However, the aforementioned ceramic capacitors have a lower equivalent series resistance (ESR) component when compared to the electrolytic or tantalum capacitors. Different from other commonly used capacitors having a low ESR component, the ceramic capacitors display a peculiar phase characteristic in which an inherently low ESR component is further decreased at a specific frequency.
As mentioned above, when a low-ESR capacitor having an extremely low ESR component is used as the phase compensation capacitor C1 as is, the phase characteristic of the driver circuit 11 is influenced, thereby creating the zero in terms of phase characteristic at a higher frequency side. As a result, the loop gain is increased thereat so that the driver circuit 11 becomes unstable, making it prone to oscillate,
The frequency at which the aforementioned zero occurs is given by   f  =      1          2      ⁢              xe2x80x83            ⁢      π      xc3x97      Co      xc3x97      ESR      
In the above equation, it is assumed that capacitance of the phase compensation capacitor C1 is Co and the equivalent series resistance component thereof is ESR.
As calculated from the equation, the frequency at which the zero occurs moves to a lower frequency side if a larger ESR value is given to the phase compensation capacitor C1. Consequently, a wider phase margin becomes available by which a possibility of oscillation is decreased.
Therefore, when a capacitor such as a ceramic capacitor having a low ESR component is used as the phase compensation capacitor C1, it is necessary to place an external resistor R1 in series so as to compensate for the low ESR component of the capacitor.
As mentioned above, if a low-ESR capacitor (a capacitor having a low ESR component) such as a ceramic capacitor having no polarity is used as the phase compensation capacitor C1 and is connected to the output terminal T1, there is no worry over a reverse connection that leads the capacitor to cause problems such as emitting smoke, thereby lowering the occurrence of troubles when mounting components and enabling a smooth flow of the capacitor mounting process.
On the other hand, a low-ESR capacitor for use as the phase compensation capacitor C1 requires the additional external resistance R1 which is not necessary if an electrolytic capacitor or the like is used instead. This requirement causes such problems to be solved as an increase in the number of components to be used, a decrease in reliability because of increased connection points, an increase in mounting area, an increase in cost, and the like.
An object of the present invention is, in light of the problems caused by conventional devices, to provide a semiconductor device that offers advantages such as a smaller mounting area even if a low-ESR capacitor is used as a phase compensation capacitor.
To achieve the above object, according to one aspect of the present invention, a semiconductor device requiring a phase compensation capacitor to be connected externally to the output terminal has an output transistor and an output terminal to which the output transistor is connected, wherein, to permit a low-ESR capacitor having a low equivalent series resistance component to be used as the phase compensation capacitor, the semiconductor device further comprises: a resistor of which one end is connected to a node between the output transistor and the output terminal; and a capacitor connection terminal to which another end of the resistor is connected, so that, when the low-ESR capacitor is connected externally to the capacitor connection terminal, the low equivalent series resistance component thereof is compensated for by the resistor.
According to another aspect of the present invention, a semiconductor device requiring a phase compensation capacitor to be connected externally to the output terminal has an output transistor and an output terminal to which the output transistor is connected, wherein, to permit a low-ESR capacitor having a low equivalent series resistance component to be used as the phase compensation capacitor, the semiconductor device further comprises: a resistor of which one end is connected to ground; and a capacitor connection terminal to which another end of the resistor is connected, so that, when the low-ESR capacitor is connected externally between the output terminal and the capacitor connection terminal, the low equivalent series resistance component thereof is compensated for by the resistor.